1. Field of the Invention
The present invention relates to a suck back valve which, for example, prevents liquid drip from occurring in a fluid supply port, by sucking back a fluid which flows through a fluid passage in accordance with a displacement action of a diaphragm, yet which is capable of stabilizing the suck back amount of the fluid.
2. Description of the Related Art
The suck back valve has been hitherto used, for example, in a production process for forming semiconductor wafers. The suck back valve has a function to prevent so-called liquid drip, i.e. a phenomenon in which a minute amount of coating liquid drips toward the semiconductor wafer from a supply port when the supply of coating liquid to the semiconductor wafer is stopped.
A suck back valve according to a conventional technique is shown by FIG. 5, further details of which may be seen, for example, in Japanese Utility Model Publication No. 8-10399.
The suck back valve 10 includes a main valve body 18 formed with a fluid passage 16 for making communication between a fluid-introducing port 12 and a fluid-discharging port 14, and a bonnet 20 coupled to an upper portion of the main valve body 18. A diaphragm 26 formed from a fluorocarbon resin like material, for example, is displaceably disposed in the fluid chamber 16. A passage 19, which serves for supply and discharge of air inside of a chamber 17 which is covered by the diaphragm 26 when the diaphragm 26 is displaced, communicates with the chamber 17. A thick-walled portion 22 is formed in the center of the diaphragm 26, and a thin-walled portion 24 is formed in surrounding relation around the thick-walled portion 22.
A protrusion 27 is formed on an upper part of the thick-walled portion 22, the diaphragm 26 being connected to the piston 30 by fitting the protrusion 27 into a cavity 29 defined in a lower part of the piston 30. A v-packing 32, slidable along an inner wall surface of the valve body 18 and serving a sealing function, is installed on the piston 30. Further, a spring 34 which normally presses the piston 30 upwardly is disposed inside the valve body 18. A pressurized air supply port 28 is formed in the bonnet 20, wherein a pressurized air supply source (not shown) is connected to the pressurized air supply port 28 through an unillustrated flow amount control valve or the like. Further, reference numeral 36 indicates an adjustment screw abutting with the piston 30, for adjusting a flow amount of a coating liquid which is sucked by the diaphragm 26, by means of adjusting a displacement amount of the piston 30.
Operation of the suck back valve 10 shall now be explained in outline. In an ordinary state in which a coating liquid is supplied from the fluid-introducing port 12 to the fluid-discharging port 14, the flow amount control valve or the like is controlled, wherein pressurized air is supplied to the pressurized air supply port 28 from the pressurized air supply source. As a result, the piston 30 is displaced downwardly in accordance with the pressure of the pressurized air, and the diaphragm 26 which is coupled to the piston 30 protrudes into the fluid passage 16, as shown in FIG. 5 by the two-dot chain line.
When flow of coating liquid through the fluid passage 16 is halted, the piston 30 and diaphragm 26 are raised in unison under an action of the elastic force exerted by the spring 34, by controlling the flow amount control valve, or the like, and stopping the supply of pressurized air supplied to the pressurized air supply port 28 from the pressurized air supply source. Along with controlling such displacement by abutment of an end of an adjustment screw 36, a predetermined amount of coating liquid remaining in the fluid passage 16 is sucked under action of a negative pressure produced by the diaphragm 26. Thus, dripping of coating liquid, which would otherwise be caused at a fluid supply port connected to the fluid-discharging port 14 is prevented.
Notwithstanding, in the suck back valve 10 of the above-noted conventional technique, when the diaphragm 26 is used for an extended period, the diaphragm 26 deforms due to repeated sucking of coating liquid inside the flow passage 16, and there is the problem that the initial condition of the diaphragm cannot be maintained, whereby the amount of coating liquid sucked by the suck back valve 10 changes, along with shortening the useful life of the diaphragm 26. Further, when as described above, the initial condition of the diaphragm 26 cannot be maintained, it becomes easy for regions to be generated in the diaphragm 26 in which liquid collects, wherein coating liquid which contacts regions in which such liquid is collected becomes deteriorated, and there is the drawback that coating liquid which flows through the fluid passage mixes with such deteriorated coating liquid, polluting the coating liquid.
Furthermore, the piston 30 and the thick-walled portion 22 of the diaphragm 26 are screwed together, wherein a clearance, caused by loosening of the screw, at a connecting region between the piston 30 and the diaphragm 26 can be generated. As a result of such a clearance, an error in the displacement amount of the diaphragm 26 results, indicating a problem that errors are generated in the amount of fluid sucked by the suck back valve.